The TCR is composed of multiple subunits that function either in antigen recognition or signal transduction. The clonotypic chains are responsible for antigen specificity but have no inherent signaling capacity. However, they associate with signal transducing molecules, the CD3 components and z family of proteins. Both CD3 and z molecules contain homologous intracytoplasmic sequences (TAMs) that couple the TCR to intracellular signal transduction pathways. The CD3 chains each contain a single TAM, whereas z contains 3 TAMs. The individual TAMs of z are not identical and it has been suggested that particular TAMs might recruit distinct signal transduction molecules. This project examines the role of z in T cell development and activation. Previously, we reported that lymphocytes from mice in which the gene encoding z was inactivated by homologous recombination express barely detectable levels of surface TCR and demonstrate a severe block in T cell development. Recent experiments reveal that the few T cells present in these mice respond poorly, if at all, to TCR engagement. To specifically investigate the role of z in TCR- mediated responses, TCR expression was restored in z-deficient mice with transgenes encoding z-variant molecules that partially or completely lacked TAMs. Analysis of anti-z immunoprecipitates from surface radioiodinated thymocytes demonstrated that the z -variant molecules, even when totally lacking functional TAMs, can dimerize, associate with other TCR components, and promote surface TCR expression. Flow cytometric analysis confirmed that z- variant transgenic proteins are able to promote surface TCR expression on thymocytes and peripheral T cells. Interestingly, each of the z variant proteins, whether they expressed 3,2,1 or 0 TAMs supported T cell development.as they promoted the generation of mature, single positive thymocytes. Moreover TCRs that contain signaling-deficient z chains were capable of transducing signals associated with thymocytes development, including increased surface CD69 expression and regulation of mRNA encoding CD5 , RAG-1 and RAG-2. Although the z chain signaling motifs were not absolutely required, their presence appeared to augment T cell development.as mice expressing full length z chain (3 TAMs) generated more mature single positive cells than those expressing a z chain lacking TAMs. Future experiments will examine the ability of z- variant reconstituted TCRs to support peripheral T cell responses and cytokine production.